Storage QuestionGeneral

Can someone explain this screen shot to me? I'm not sure why my available space plus what's been used don't add up to the 13GB available. I know the OS takes space, and that's why the available is 13GB and change, but what about the rest?

What's even more confusing is that when I click the Apps (3.92GB) it then says that there are 4.1GB used of my Internal Storage and 9.1GB free. What?